Remove assertion about job_type

If the code is buggy an `UnboundLocalError` will be thrown during the
job creation

Author: vkarak

reframe/core/pipeline.py

@@ -1665,8 +1665,6 @@ def _create_job(self, job_type, force_local=False, **job_opts):
             script_name = 'rfm_build.sh'
         elif job_type == 'run':
             script_name = 'rfm_job.sh'
-        else:
-            assert 0, "[internal error] unknown job type"
 
         return Job.create(scheduler,
                           launcher,